Platform: add missing std._Private modules

Add the missing `std._Private.xstddef`, `std._Private.yvals`, and
`std._Private.yvals_core` modules.  This repairs the use of `std.map` as
a private member variable of a type with an explicit ctor.

Author: compnerd

stdlib/public/Platform/vcruntime.modulemap

@@ -707,6 +707,10 @@ module std [system] {
       export *
     }
 
+    explicit module xstddef {
+      header "xstddef"
+      export *
+    }
 
     explicit module xstring {
       header "xstring"
@@ -727,5 +731,15 @@ module std [system] {
       header "xutility"
       export *
     }
+
+    explicit module yvals {
+      header "yvals.h"
+      export *
+    }
+
+    explicit module yvals_core {
+      header "yvals_core.h"
+      export *
+    }
   }
 }